With regards to the XPS Thunderbolt 3 PCIe speed running at 2x rather than the advertised 4x, the performance loss vs. the Late 2016 13" MacBook Pro is about 15% in Nvidia Optimus mode. Time Spy results through AKiTiO Node + GTX 980 Ti:

Excellent question. I did not update it through the Dell XPS 9350 but I'm certain the Node firmware update would work if I did.

The XPS 9350 has always been able to connect to the thunderbolt enclosure itself just fine. It's the incompatible firmware between the Node and XPS which triggers the message "External GPU Support: No". This message it still there after the Node firmware update but that doesn't matter as long as the eGPU shows up in Device Manager. The only way for this to disappear is when Dell releases a new Thunderbolt firmware update for this XPS 9350.